Name: jn10789 Date: 01/20/99


JDK1.2 includes a connect(InetAddress addr, int port) call in class
java.net.DatagramSocket. This is only really useful for one thing,
which is to throw an Exception when the VM receives an ICMP error
message saying that the address was invalid, for example because
no process was listening on the specified port.

Quoting from "Unix Network Programming" [Stevens 1990, p271]:

"...the Internet protocols specify that a host should generate an
ICMP port unreachable message if it receives a UDP datagram specifying
a UDP port for which no process is waiting to read from. The host that
sent the UDP datagram and receives this ICMP message can try to
identify the process that sent the datagram, and notify the
process. 4.3BSD, for example, notifies the process with a "connection
refused" error (ECONNREFUSED) on the next system call for this socket,
only if the process had connect()'ed the socket to the destination
address."

Having this behaviour is essential when implementing certain
protocols on top of UDP, because relying only on timeouts takes
too long. The Session Initiation Protocol (SIP) is an example of
such a protocol.

The following code generates 5 UDP messages and SHOULD fail with
an exception on the second send(). I've tested it on NT4.0 with no
exception being thrown. This is when the remote address is
either an NT4.0 or an HP-UX host. It may be the case that NT4
doesn't generate the ICMP error but I'm assuming that HP-UX does.

Maybe the problem is that NT4 doesn't pass on those ICMP error
to the sending process, in which case running the test program
on a Solaris box should work (ie result in an exception).


import java.net.*;

public class UdpConnect {
    public static void main(String[] args) throws Exception {
        int port = 7000;
        String host = "127.0.0.1";
        int i;

        for (i = 0; i < args.length; i++) {
            if ("-p".equals(args[i])) {
                port = Integer.parseInt(args[++i]);
            } else if ("-h".equals(args[i])) {
                host = args[++i];
            }
        }
        InetAddress addr = InetAddress.getByName(host);
        DatagramSocket sock = new DatagramSocket();
        byte[] buf = "Hello, server!".getBytes();
        DatagramPacket p = new DatagramPacket(buf, buf.length, addr, port);
        int nsend = 0;
        
        sock.connect(addr, port);
        
        for (i = 0; i < 5; i++) {
            System.out.print(".");
            sock.send(p);
            try {
                Thread.currentThread().sleep(1000);
            } catch (Exception ex) {
                // if no server at other end we SHOULD get an ECONNREFUSED
                // exception here on second send()
                ex.printStackTrace(System.err);
            }
        }
    }
}
(Review ID: 52238)
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